The present invention relates to a marine lubricant for a two-stroke engine with improved neutralization properties with respect to the sulphuric acid formed during the combustion of fuels, in particular fuel oils, said to have a high sulphur content, that is to say greater than 1% by weight. It relates more particularly to a lubricant exhibiting an improved neutralization capacity for sulphuric acid with respect to that of the marine lubricants on the market.
The marine oils used in slow-speed two-stroke crosshead engines are of two types. The cylinder oils, on the one hand, provide for the lubrication of the piston-cylinder assembly and the system oils, on the other hand, provide for the lubrication of all the moving parts outside the piston-cylinder assembly.
The matter under consideration here is the cylinder oils, for which the importance of the neutralization of the sulphuric acid is vital as they lubricate the part of the engine where combustion of the fuel takes place. In this part, the combustion residues, the acidic gases and the lubricating oil are in contact.
It is usual to characterize an oil by its BN or Base Number, which characterizes its basicity. It is measured, in the majority of cases, according to ASTM Standard D2896 and is expressed in equivalent by weight of potassium hydroxide per gram of oil or mg of KOH/g. The BN, until now, was a criterion of choice which makes it possible to adjust the basicity of the oil to the sulphur content of the fuel oil used, in order to be able to neutralize all the sulphur present in the fuel capable of being converted to sulphuric acid by combustion. Thus, the higher the sulphur content of a fuel oil, the higher the BN of a marine oil should be. This is why marine oils with BN values varying from 5 to 100 mg KOH/g are found on the market.
However, the Applicant Company has noticed that the BN is not representative of the neutralization phenomenon itself, as it takes place in the film of oil which provides for the lubrication in the piston-cylinder assembly. Thus, even for oils exhibiting a sufficiently high BN, corrosive wear phenomena related to the attack by sulphuric acid on the metal walls of the components concerned are commonly observed in the piston-cylinder assembly of the engines. The assessment of the neutralization cannot be made by the BN alone of the oil, as this parameter does not make it possible to predict the effectiveness of the basic additives present in the cylinder lubricants with respect to sulphuric acid condensed in the oil which, as has been demonstrated by A. Van Helden (xe2x80x9cA Physico-Chemical Model of Corrosive Wear in Low Speed Enginesxe2x80x9d, Report D-9, CIMAC 1987), involves diffusion processes which vary according to the chemical nature of these additives.
Thus, although the oils are nonpolar hydrocarbons, it is found that the acidic products from the combustion of the fuel oils, which are polar in nature and therefore difficult to dissolve in hydrocarbons, diffuse into the oils. This diffusion phenomenon varies according to various factors, such as the oil throughput, the rotational speed of the engine and the wettability of the metal surface. All these factors as well as the BN are important in predicting the corrosive wear by sulphuric acid. However, in order to prevent this corrosion, two factors are controllable by the manufacturers of lubricants, the BN and the wettability over the metal wall, which both depend on the chemical composition of the additives in the lubricating oil.
The Applicant Company has discovered that, in order to prevent corrosion, it is necessary to know the rate of neutralization of the sulphuric acid, diffused into the lubricant, by the basic compounds of the additives present. This is because, in order to be effective, these basic compounds have to react with the sulphuric acid before it reaches the metal walls of the cylinder and mainly in the film of oil formed in the cylinder where the corrosion phenomenon is greatest.
To achieve the desired basicity in the lubricants, that is to say a satisfactory BN, several additive compositions are recommended by the manufacturers of lubricants for marine engines.
Some of these manufacturers provide additive compositions including a mixture of detergents of phenate, salicylate and sulphonate types, such as disclosed in Patent EP 0,662,508 for low- and medium-speed diesel engines. Others recommend, for marine engines, lubricants comprising mixtures of overbased detergents of sulphonate type and of phenate type, the ratio by mass of which varies from (1:1) to (15:1), and which additionally include an ash-free borated dispersant of succinimide type, as disclosed in Patents EP 0,331,359 and U.S. Pat. No. 4,842,755.
The Applicant Company has unexpectedly found that a specific combination of detergent and dispersant additives makes it possible to improve the ability of the lubricant to neutralize the sulphuric acid produced during the combustion of sulphur-comprising hydrocarbons.
In fact, the present invention is targeted at a lubricant having a sufficient basicity and a sufficient speed of neutralization of the sulphuric acid formed to prevent the corrosive wear phenomena observed in the cylinders of slow-speed two-stroke crosshead engines.
A subject-matter of the present invention is therefore a lubricant for a slow-speed two-stroke crosshead marine engine, fed with a fuel with a sulphur content of greater than or equal to 1%, composed of a mixture of distillates used as lubricating bases including x% by weight of dispersant additive of succinimide type, y% by weight of overbased detergent additive of sulphonate type and z% by weight of overbased detergent additive of phenate type, characterized in that x, y and z are chosen within the ranges 0.5xe2x89xa6xxe2x89xa62.5, 3.5xe2x89xa6yxe2x89xa610 and 11xe2x89xa6zxe2x89xa624.5 such that 15xe2x89xa6x+y+zxe2x89xa636 and 1.5xe2x89xa6y/xxe2x89xa613, in that the BN of the lubricant, determined according to ASTM Standard D2896, is greater than or equal to 70 mg of KOH/g and in that the rate of neutralization by the lubricant of the sulphuric acid has a value greater than or equal to 11xc3x97102 Pa/s at 100xc2x0 C., the rate of neutralization being defined by the maximum rate of increase in pressure in a closed chamber with walls covered with a film of the said lubricant with a thickness of 80 to 150 xcexcm, which chamber is maintained at 100xc2x0 C. and into which is introduced a known amount of sulphuric acid.
The advantage of the method used to measure the rate of neutralization of the sulphuric acid by the lubricant is to be able to reproduce the film of oil present in the cylinder part of the marine engine and the phenomenon of reaction of the sulphuric acid with the overbased detergents in the presence of dispersant and thus to approach a novel and more appropriate characteristic of the neutralizing capacity of these compositions. A more detailed description of the method used is given in Example I below.
By means of this device, the Applicant Company has found that, in order to promote the increase in the rate of neutralization, the said lubricant necessarily has to include a sufficient amount of a mixture of at least one overbased detergent of sulphonate type with at least one dispersant of succinimide type in order to prevent phenomena of corrosion by the sulphuric acid and that this mixture is synergistic. The Applicant Company has made a specific selection of a mixture of additives in order to obtain a lubricant capable of rapidly neutralizing any acid invasion of the film formed inside the cylinder part of the motor, which is where acidic corrosion is generally observed.
In a preferred form of the invention, the y/x, overbased detergent of sulphonate type/dispersant of succinimide type, ratio by mass preferably varies from 1.5 to 5.
Among the accessible detergents, the preferred overbased detergents of sulphonate type are chosen from the group consisting of neutral and basic alkali metal and alkaline earth metal salts of petroleum sulphonic acids and of long-chain alkylarylsulphonic acids with BN values, determined by ASTM Standard D2896, varying from 300 to 500 mg of KOH/g. In a preferred embodiment, these sulphonate compounds are alkaline earth metal alkylarylsulphonates and preferably calcium mono- and dialkylarylsulphonates, each alkyl group comprising from 12 to 18 carbon atoms and the aryl group being a benzyl, a tolyl or a xylyl.
These overbased detergents of sulphonate type are effective with regard to the neutralization of the sulphuric acid in the lubricant film when they are taken in combination with at least one dispersant of succinimide type, this dispersant being chosen from the group consisting of succinimide derivatives from the group comprising polyalkenylsuccinimide borates, in which compounds the succinic acid or anhydride derives from a polymer of a C3 or C4 olefin, preferably from polyisobutylene with an average molecular mass of between 1000 and 3000, this acid or anhydride having reacted with a polyalkylenepolyamine from the group consisting of diethylenetriamine, triethylenetetramine, tetraethylenepentamine and their mixture, and in which compounds, for each polyalkenyl group, correspond from 1 to 2 succinimide groups.
However, in order to provide the lubricant with sufficient thermal resistance and to limit its oxidation, the lubricant comprises an overbased detergent of calcium phenate type, with a BN, determined by ASTM Standard D2896, varying from 50 to 400 mg of KOH/g, chosen from the group consisting of alkali metal and alkaline earth metal phenates of alkylphenols and of alkylmercaptophenols, the linear or branched alkyl groups comprising from 4 to 50 carbon atoms, preferably from 9 to 15 carbon atoms, taken alone or as a mixture.
Preference is given, among the detergents of phenate type of the invention, to a mixture of 95 to 100% by weight of calcium alkylmercaptophenates, the alkyl group comprising from 9 to 15 carbon atoms, with a BN varying from 200 to 400 mg of KOH/g, and of 0 to 5% by weight of a calcium alkylmercaptophenate with a BN of less than 150 mg of KOH/g, the alkyl group comprising from 9 to 15 carbon atoms.
In a preferred embodiment of the invention, for maximum effectiveness with regard to the neutralization of the sulphuric acid in the film, the said marine engine lubricant will advantageously include x% by weight of polyisobutylenesuccinimide, comprising one polyisobutylene group per 1.1 to 1.8 succinimide groups, y% by weight of calcium sulphonate with a BN of between 300 and 500 mg of KOH/g, z% by weight of a mixture of calcium alkylmercaptophenates and 64 to 85% by weight of a mixture of hydrocarbons composed of lubricating mineral bases with densities at 15xc2x0 C. varying from 850 to 950 kg/m3 known to a person skilled in the art.
The lubricant according to the invention can additionally comprise from 0 to 1% of other additives, in particular of an antifoaming additive.
Examples are given below targeted at illustrating the invention without, however, wishing to limit the scope thereof.